The present invention relates to a monitor for use with an ungrounded power distribution system. The monitor detects any decrease in the completeness of the system's isolation from ground by indicating the magnitude of current that would pass through a grounded low impedance connected to a system conductor due to the existence of either balanced or unbalanced resistive, capacitive or hybrid system leakages.
Isolated ungrounded power distribution systems are required under the standards set forth by the National Fire Protection Association Publication, No. 56(a)--1971, which is part of the National Electric Code, Article 517, in hospital rooms where anesthetic is administered such as operating rooms and special purpose rooms where electronic devices are attached to the patient e.g., cardiovascular laboratories. Furthermore, the standards require that a monitor be used with such ungrounded systems to provide continuous indication that the system remains safely isolated from ground. The present invention is a dynamic line isolation monitor, LIM, specifically adapted for use in hospital ungrounded power supply systems.
In an ideal isolated ungrounded system, there is infinite impedance to ground so that if a direct low impedance connection were made between any conductor and ground no current will flow through the low impedance. However, all isolated ungrounded electrical distribution systems experience a certain amount of leakage to ground. Paths of resistive leakage or faults are caused by the fact that no insulator is perfect and insulation deteriorates with age and use. Paths of capacitive leakage or faults, the more severe problem, are inherently present in all systems and increase with the length of the wire conductors. Both types of leakages increase with the number of user devices connected to the system. Naturally, any supply conductor may have a combination of both resistive and capacitive leakage paths to ground. These leakages in an actual isolated ungrounded system provide a path to ground and if a grounded low impedance were connected to a conductor the current, now having a return path due to the leakages, would flow through the low impedance. The total current that would flow through a low impedance if it were connected between an isolated conductor and ground is defined as a hazard current. The fault hazard current is defined as the hazard current of a given isolated system with all user devices connected except the line isolation monitor. The monitor hazard current is defined as the amount of hazard current due solely to the connection of the monitor to the ungrounded supply system. The total hazard current is the combination of the fault hazard current caused by electrical devices connected to the isolated system plus the monitored hazard current due to the monitoring device connected to the isolated system. The NFPA standards require that if the total hazard current exceeds a predetermined value, a red light and audible warning alarm be energized. Furthermore, the standards limit the amount of monitor hazard current contributed by the LIM to the total hazard current.
One reason for the NFPA standards is the safety of the patient. If the normal high body resistance of a patient is in any way bypassed then he becomes electrically susceptible. The degree of electrical susceptibility can vary depending on the medical procedure being followed. If the procedure requires that the patient have an electric probe or catheter connected directly to the heart muscle, the electrical susceptibility is at a maximum and a small amount of current passing through the patient is extremely dangerous. The electrical susceptibility of the patient is reduced if the medical procedure only requires electrodes connected to the outer skin after a conductive paste has been applied to the point of contact.
In either situation if the human body forms a low impedance due to the bypassing of its normally high resistance and then forms a connection between a line conductor and ground, the total hazard current will flow through the body due to the return path formed by the leakage or faults in the system. Alternating current passing through the human body with a magnitude exceeding 2 milliamperes may produce extreme shock and possibly death. The line isolated monitor used with an ungrounded isolated power distribution system in a hospital continuously indicates the worst or maximum total hazard current that would flow through the human body in such a condition.
Prior art monitors include the static ground detector which utilized a low value resistance placed between one side of the conductor and ground or simply a balanced static bridge circuit which detected any unbalanced impedance condition between the lines. The primary difficulty with this type of static detector was its failure to detect simultaneously occurring connections of resistive or capacitive leakages to ground on each conductor. Prior art dynamic ground detectors overcame the static detectors insensibility to unbalanced faults. The prior art dynamic detector is connected at predetermined intervals between each conductor of the supply and ground to determine the current flow in the detector path. If this current exceeded a predetermined magnitude, the detector indicates an unsafe grounding condition. These types of dynamic line monitors or ground detectors contribute a high amount of monitor hazard current to the total hazard current, produce large spikes in the conductor line and introduce noise interference due to the alternate and periodic connection of the detector to one line and then to the other.